In the isolated and perfused retina of the toad, intracellular work will be conducted in the outer segments of red rods. The effects of changing ionic composition of the perfusate will be studied, to elucidate the ionic mechanisms that control membrane potential, the light-evoked receptor potential, and receptor sensitivity. Particular attention will be given to Ca2 ion and Ba2 ion, which have been shown to exert strong effects, and which are both indicated by prior evidence to be concentrated in the outer segments of vertebrate photoreceptors. Our newly developed microelectrode techniques will be applied to the isolated rabbit retina, with the goal of systematic intracellular recording in mammalian photoreceptors. Attempts will be made to study the basic properties of receptor potentials in the rabbit retina, including response form and amplitude as a function of stimulus intensity, spectral sensitivity curves, and the types of receptor interactions that may be demonstrated. In snapping turtle, intracellular recording will be conducted to determine, for each major type of horizontal cell, the receptor types that feed into that cell and the pathways through which these inputs operate. An attempt will be made to correlate these results with previous anatomical studies of horizontal cell inputs in the snapping turtle. Additional anatomical work will be undertaken, as required, and Procion Yellow will be used to identify cells recorded.